1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods and apparatus for reducing dielectric breakdown during operation. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for the shock processing of solid materials typically involve the use of high explosive materials in contact with the workpiece. High explosive materials or high pressure gases are used to accelerate a plate that strikes the solid to produce shock waves therein. Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces.
Shock processing with coherent radiation has several advantages over what has been done before. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on preselected surface areas and the operating mode is easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to shock the workpiece repetitively.
Laser peening (sometimes referred to as laser shock processing) utilizes two overlays; a transparent overlay (usually water) and an opaque overlay (usually an oil-based or acrylic-based black paint). During processing, a laser beam is directed to pass through the water overlay and is absorbed by the opaque overlay, causing a rapid vaporization of the this overlay and the generation of a high-amplitude shock wave. The shock wave cold-works the surface of the part and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
A problem, in utilizing transparent overlays, is that dielectric breakdown of the overlay may occur during use. The term dielectric breakdown, as used in this application, is the laser-induced ionization of a transparent overlay and subsequent formation of a plasma. It occurs during laser peening, when the laser intensity incident on the transparent overlay causes ionization of the transparent overlay which subsequently produces an avalanche of free electrons. As a result, a plasma forms that absorbs incoming laser light, and thereby reduces the amount of laser energy that can reach the opaque overlay. This significantly reduces the peak pressure and duration of the shock wave incident on the workpiece that is being processed.
In the case of a linear polarized laser field, experimental data show that the dielectric breakdown is a probabilistic event that depends on the rms laser field, E, through the simple relation P=exp (-K/E), where K is a proportionality constant.
The net result of the dielectric breakdown is that the laser energy is not efficiently coupled into the sample surface of the workpiece. Dielectric breakdown may occur in localized sites within the dielectric material (transparent overlay), resulting in a number of small ionized spots that are randomly scattered throughout the overlay. At these spots, a localized plasma forms which absorbs energy from the process area.
What is needed in the art is a laser shock process that is highly repeatable without irregularities and reduction in the applied pressure.